All manner of magical and mythological creatures reside here. You’ll find sprites, fairies, satyrs, dryads, etc in this realm.
Many creatures are looking to make deals, such as deals for how a mortal can get out of the Demi Plane. Some residents of the Demi Plane would like to travel to the Mortal Plane for some reason or another, but need a mortal to summon them from the one plane to the other. So, a deal might be struck for the mortal to be shown a way out of the Demi Plane in exchange for said mortal summoning the helpful resident of the Demi Plane when that mortal returns to the Mortal Plane.
Most beings of the Demi Plane seem restricted to it, although some do make their way into the Mortal Plane from time to time whether by happenstance, or because some do know the way between the planes.
Denizens of the Demi Plane fall into one of two broad categories: those that are specific legends versus those that are part of species that is said to live in/be native to the Demi Plane. Specific legendary figures/beings/creatures tend to be functionally immortal. An example would be Lady Isae. Beings/creatures that are part of a specific species, unless otherwise stated in some specific story/myth, may be short or long-lived, typically mortal-born or must somehow come into being thru either natural or magical means, age, and die. Examples of these types of creatures would be satyrs, fairies, orcs, goblins, etc.
How do you kill a legend? The gods found that the only way to truly destroy a specific legendary being was to throw it into the Mortal Plane – this before the Mortal Plane was shut off by the gods. Once in the Mortal Plane the legendary being could be permanently killed, albeit still have it’s powers. The beings of the Demi Plane do not inherently know this unless this information is part of their story/legend or have learned it while living in the Demi Plane.